Fabrication of neutron sources



United States Patent FABRICATION 0F NEUTRON SOURCES John H. Birden,Dayton, Ohio, assignor to the United States of America as represented bythe United States Atomic Energy Commission No Drawing. ApplicationJanuary 11, 1956 Serial No. 558,597

4 Claims. (Cl. 250-845) The present invention relates to methods forpreparing neutron sources, and more especially to improved methods forpreparing neutron sources from polonium- 210 and boron. This inventionespecially relates to an improved method for fabricating polonium-boronneutron sources to achieve substantially higher efiiciencies in neutronproduction than when utilizing the methods and techniques described inmy co-pending application S. N. 542,117(48), filed October 21, 1955.

Neutron sources of the prior art have commonly been formed from radiumand beryllium. Such sources are prepared by slurrying finely groundberyllium powder in an aqueous solution of radium bromide, evaporatingto dryness, and compressing the residual powder into a pellet. Thepellet must be placed in a gas-tight container to confine the radondecay product. Sources thus produced are not entirely satisfactorybecause their intensity changes continuously over the first few weeksafter preparation, due to accumulation of radioactive decay products ofradium. More important, such sources produce a high gamma-ray hazard,since one curie of radium produces .05 roentgen in 3.5 minutes at adistance of one meter.

To reduce dangerous gamma activity, sources have been prepared usingpolonium-210 with a neutron emitter such as beryllium or boron bymethods described in my co-pending application, supra. While the neutronyield is much less than that of a radium source, the dangerous gammaradiation is reduced by a factor of In addition, the amount of poloniumper curie is so small that self-adsorption of alphas is negligible,compared to that in radium. Moreover, no radioactive gas similar toradon is formed from the polonium.

In the novel method of source preparation disclosed in my co-pendingapplication, supra, the boron or beryllium is reduced to a finely groundpowder and inserted in a metal container, the polonium-210 is insertedin the container, the container is sealed, and the sealed container isheated at an elevated temperature for a time sufficient to distributethe polonium over he surface of the neutron emitting powder, the heatingis terminated, and the source is allowed to cool. When beryllium isemployed as the neutron emitter, the heat treatment causes the poloniumto unite chemically with the berryllium, so that upon cooling, thepolonium remains disposed throughout the powder, and the sources thusproduced remain of constant intensity. I have found, however, that boronsources decrease substantially in intensity upon cooling, and are thenrelatively inefiicient. I have postulated that the polonium tends toleave the boron and deposit on the metal container, so that it does noremain distributed uniformly throughout the boron. While such relativelyinefficient boron sources are nevertheless useful, it is highlydesirable to improve their neutron-producing efiiciency to reduce thecost and size of sources relative to the neutron flux produced.

With a knowledge of the difficulties associated with 2,870,339 PatentedJan. 20, 1959 production of neutron sources with boron in the prior art,it is in object of the present invention to provide an improved methodfor fabricating neutron sources employing boron. Another object of myinvention is to provide an improved method for farbication ofpoloniumboron neutron sources. Yet another object of the invention is toprovide an improved method of fabrication of neutron sources includingthe volatilization of polonium onto boron, by which method the poloniumis prevented from detaching itself from the boron upon cooling with theresulting weakening of the strength of the source. A primary object ofthe invention is to provide a method for fabricating boron neutronsources including the steps of first coating the boron powder with anelement capable of forming a polonide, then volatilizing polonium ontothe coated powder by a slow heating process, and cooling the sourceafter the polonium has united chemically with the coating material, bywhich method the polonium remains distributed uniformly throughout theboron powder and the neutron flux remains substantially at the maximumvalue.

These and other objects of the present invention will become apparentfrom the following detailed descriptions of preferred methods forpreparing neutron sources in accordance with the teachings of myinvention.

In practicing my invention, the source materials are sealed inside acontainer, which may be of any convenient shape, but cup-shapedcylinders having plugs in their open ends are preferred. The containersshould be fabricated from materials which are not adversely affected bythe source-fabricating operations, which will not combine chemicallywith substantial quantities of polonium, and to which polonium does notadhere readily. I prefer to fabricate containers from cold-rolled steel,although, wrought-iron and other suitable materials may be used.

The polonium may be introduced into the container by first plating itupon a metal foil and then placing the foil inside the container.Alternatively, the polonium may be deposited on the interior walls ofthe container by volatilization techniques. For example, the opencontainer may be placed in a vacuum chamber in spaced relation with thefoil containing the polonium. While the foil is heated, as by induction,the container is cooled, as by a cold finger, below the temperature ofthe foil so that the polonium will transfer from the foil to the insidewalls of the container.

Boron powder is prepared for use in the source by first forming a thincoating uniformly throughout the powder of a substance capable offorming a polonide and also characterized by good adherence to theboron. The following materials meet these requirements: nickel,platinum, and silver. Other materials may be found which are suitable toa lesser degree. The coating may be formed in any conventional mannersuch as by evaporation, sputtering, chemical reduction, or the like.When the boron powder is coated with silver, I prefer to use thewell-known Rochelle salt mirror process. Platinum is preferablydeposited by repeated volatilizations of the platinum off a hot tungstenwire in a vacuum system. Nickel coating may be done by thermaldecomposition of nickel carbonyl in a vacuum, as in the containersealing operation below. The amounts of coating material required areonly slightly in excess of the amounts theoretically required to formthe polonides, so that only a thin coat is formed.

The container may be sealed by applying a coating of nickel to the outersurface. This is preferably accomplished by placing the container in anatmosphere of nickel carbonyl and heating it by induction to'the desiredtemperature. A coating of 0.02 inch of nickel ond when the source wasallowed to cool.

has proved satisfactory to insure an impervious seal and to preventescape of radioactive material from the interior. Other coatingmaterials or other coating thicknesses may be employed, neither thematerial nor the coat thickness being critical in source productions, solong as the container is sealed.

The source may be heated by an induction heater or other furnaceprovided with temperature control means in a vacuum or in an inertatmosphere, such as helium, to prevent oxidation. The temperature shouldbe monitored by a. thermocouple and the neutron flux by a neutron ratemeter. The temperature is increased very slowly, allowing slowvolatilization of the polonium off the foil or container walls. Theheating should be stopped when the desired flux reaches a maximum andbegins to level off. This may be accomplished by an operator manually orthrough a switch responsive to recorder pen movement, for example. Thecontainer is heated to a temperature sufiicient to cause volatilizationof the polonium inside the container, causing it to distribute uniformlythroughout the boron powder and unite chemically with the coatingsubstance, volatilization occurring at temperatures above about 450 C.If the source does not reach the desired strength upon a first heating,it may be allowed to cool and then heated again.

The following examples illustrate some of the permissible variations andalternatives in preparing neutron sources in accordance with my novelmethod.

Example I Substantially 5 grams of 230 to 325 mesh (U. S. standardsieve) boron powder was coated with silver by the Rochelle salt process.About 0.014 gram of silver per gram of boron powder was deposited. 1.8grams of boron powder coated with silver, was placed in a coldrolledsteel container, .66 inch in outer diameter and .66 inch high. About3.46 curies of polonium-210 was volatilized into the source container ina vacuum, the silver coated boron powder was added to the container, andthe container was closed. The container was sealed by heating in nickelcarbonyl at a pressure of approximately 3 cm. Hg. Heating was continuedand the nickel was deposited until the external dimensions reached .7inch in diameter and .7 inch in height, at which time the source wasremoved from the nickel atmosphere. The source was monitored with aneutron counter and produced a total neutron fiux of 1.145 neutrons persecond.

The neutron efficiency of this source was then 40.65%, based on a thicktarget yield for boron of 22 neutrons per 10 alphas, which is equivalentto 8.14 10 neutrons per second per curie of polonium at 100% etficiency.

The source was then placed in a vacuum system and heated in helium at apressure of 10 cm. Hg. The temperature and the neutron flux weremonitored and continuously recorded while the neutron source was heatedby induction. The temperature of the source was increased slowly andgradually to about 500 C., and maintained there until the flux reached amaximum value of l.891 10 neutrons per second, after which the sourcewas cooled. The neutron efiiciency had then increased to 67.14%.

Example II A source was prepared in exactly the same way as that inExample I, except that the boron powder was uncoated. The neutron fluxrecorder chart indicated that the neutron flux produced by the sourceincreased with increase of temperature in the manner above stated toabout 1.9 10 neutrons per second, but it returned to the original valueof about 1.1)(10 neutrons per sec- The source of Example I maintainedits increased neutron flux of about 1.9 10 neutrons per second evenafter cooling.

Comparison of these two otherwise identical sources indicates that theadditional coating step provided in my novel method was required inorder to maintain good uniform distribution of the polonium throughoutthe powder, and here increased the neutron flux by over 70% of otherwiseidentical sources.

Example Ill Platinum was volatilized from a tungsten wire and depositedon a thin layer of boron powder, to mesh (U. S. standard sieve), in ahigh vacuum. About .0014 gram of platinum per gram of boron powder wasapplied. One gram of uncoated powder was mixed with one gram of thecoated powder. The source was then fabricated as in Example I. When thesource was sealed, a neutron fiux of 1.85 10 neutrons per second wasindicated, 4.995 curies of polonium having been used, indicating aneutron efiiciency of 45.5%. The source was then heated to about 510 C.and the flux reached 3.22 10 neutrons per second before leveling off. Aneutron efficiency of 79.2% was indicated. The flux did not return toits previous level but maintained its strength, substantially 70%greater than before the heating step.

It will be apparent to those skilled in the art that I have devised animproved method of fabricating neutron sources from polonium-210 andboron which greatly increases the neutron efficiency of these sources.From the foregoing examples, it may be seen that sources producingsubstantially 70% higher neutron fluxes than those prepared according toprior methods with identical amounts of polonium and boron may befabricated according to my improved method.

Having thus described my invention, I claim:

1. A method for preparing a source of neutrons comprising the steps ofinserting in a container a quantity of polonium-210, coating boronpowder with a material chosen from the group consisting of silver,platinum, and nickel, inserting said coated powder in said container,sealing said container to prevent escape of the enclosed material, andheating said container to a temperature above substantially 450 C. for aselected time interval to effect chemical combination of said poloniumwith said coating material.

2. A method for preparing a source of neutrons comprising the steps ofcoating uniformly a quantity of boron powder with silver, introducingpolonium into a metal container, introducing said coated power into saidcontainer, sealing said container to prevent escape of the materialstherein, slowly raising the temperature of said source to abovesubstantially 450 C. and continuously heating said source to volatilizesaid polonium, indicating the neutron flux from said source, terminatingsaid heating step responsive to said flux indication after said fluxceases to increase.

3. A method for preparing a source of neutrons comprising the steps ofinserting in a container a quantity of polonium-210, contacting aquantity of boron powder with nickel by thermal decomposition of nickelcarbonyl, inserting said coated powder in said container, sealing saidcontainer with a nickel coating, slowly raising the temperature of saidcontainer to above substantially 450 C., monitoring and indicating theneutron flux emitted by said source, and terminating said heating stepwhen said neutron flux reaches a selected level.

4. A method for preparing a source of neutrons comprising the step ofvolatilizing a quantity of polonium- 210 into a container, vaporizingplatinum from a hot filament onto a quantity of boron powder, placingsaid powder in said container with said polonium, sealing said containerto prevent escape of the enclosed material, measuring the neutron fluxemitted by said source, slowly raising the temperature of said containerto above substantially 450 C. by induction heating, and termimating saidheating step regonsive to said neutron flux OTHER RE FERENCES reaching aselected level TID-5087, U. S. Atomic Eoergy Commission docu- ReferencesCited in the file of this patent E g dated July 1952: declasslfied V.29, 1955, pp. UNITED STATES PATENTS 5 The Reactor Handbook, vol. I,AECD-3645, Atomic 2,440,999 Anderson May 4 1943 Energy Commissiondeclassified edition, Feburary 1955, 2,573,069 Segre Oct. 30, 1951 PP-3,4,5-

2,592,115 Carroll Apr. 8, 1952

1. A METHOD FOR PREPARING A SOURCE OF NEUTRONS COMPRISING THE STEPS OFINSERTING IN A CONTAINER A QUANTITY OF POLONIUM-210, COATING BORONPOWDER WITH A MATERIAL CHOSEN FROM THE GROUP CONSISTING OF SILVER,PLATINUM, AND NICKEL, INSERTING SAID COATED POWDER IN SAID CONTAINER,SEALING SAID CONTAINER TO PREVENT ESCAPE OF THE ENCLOSED MATERIAL, ANDHEATING SAID CONTAINER TO A TEMPERATURE ABOVE SUBSTANTIALLY 450*C. FOR ASELECTED TIME INTERVAL TO EFFECT CHEMICAL COMBINATION OF SAID POLONIUMWITH SAID COATING MATERIAL.